The present invention relates to toner imaging systems of the type wherein a latent charge image is developed with a pigmented toner, and the developed image is transferred to a receiving member to make a printed image. There exist many technologies for forming a latent charge image, including optical image projection onto a charged photoconductive belt or drum; charging a dielectric member with an electrostatic pin array or electron beam; and charge projection from a so-called ionographic print cartridge or from a plasma generator. Once a latent image is formed, the latent image may be transferred to an intermediate member before development, or may be developed on the same member as that on which it is formed, with different system architectures having evolved to address different process priorities, such as cost, speed, preferred type of toning system or intended receiving substrate. The toner may be of a liquid-carried or a dry powder type; the former pose environmental concerns of solvent or carrier management, especially when printing on so-called plain, or bond, papers, while the latter developers raise concerns of dust control, especially as the toner particle size becomes finer. In either case, one must generally also address problems related to erasing or cleaning intermediate image carriers, and fixing the final image.
In general, the toned images, once transferred to a receiving member require heating to dry or fix the final image, but cannot endure heat at an earlier stage, when the toner is applied, as a dust or liquid suspension, to the latent charge image. Furthermore, at an even earlier stage, heating is generally also to be avoided on or near any photoconductive elements. Even for charge deposition systems in which an electric charge is applied to a dielectric rather than photoconductive member, heat may impair the dielectric properties of some common image-holding materials.
Thus, a complete imaging system often benefits from, and may require, having the image transferred one or more times before the final printing step in order to isolate the chemicals, temperatures or other environment of one imaging process station from those of another.
Another factor which has assumed prominence in imaging systems of the foregoing type is the heat transfer, or transfusing, of a toner image onto a final receiving substrate. In various prior art constructions, the toned image is simultaneously transferred to and fixed on the final member in a melted or fused state. It may further be necessary to control the precise temperature to vary the relative tackiness or the self-adherence of the heated toner, for example, in order to achieve optimal transfer of the image between rollers, or, when transferring to a final recording sheet, in order to optimize image reflectance properties. U.S. Pat. No. 3,554,836 of Steindorf describes a general approach useful in such multi-transfer systems. According to that patent, intermediate rollers may be formed of a silicone elastomer, and transfer is efficiently arranged between two successive image-carrying members by controlling the temperature of the colored image layer so that it is in a rubbery state, while the members have surfaces of silicone elastomer of increasing energy to enable relatively effective transfer of the heated image material from one roller to the next.
Nonetheless, the transfer of a toner image from one member to another remains highly dependent on the materials used, as well as on the characteristics of the transfer nip and the speed of contact, among other variables. When one or more of these variables is selected based on independent considerations, it may prove difficult to then achieve a suitable transfer speed or efficiency with the selected variable.
It would therefore be desirable to achieve a printing system in which transfer of a toned image is quickly and efficiently effected.
It would also be desirable to achieve such a printing system wherein multiple toned images are successively transferred to form a multicolor image.